There is no such thing as a 'diffusion-type process.' The blur of a lens is simply a convolution with the PSF of the lens. If the PSF is exactly known, it can be exactly inverted with a deconvolution.

So what is a "diffusion-type process" ?

One that resembles diffusion in a mathematical sense.

If you believe lens blur "resembles diffusion in a mathematical sense," can you please give a diffusion process, as per the links you gave, which has a PSF which is a solid octagon? It is trivial to design an optical system with this (I'll use your made-up terminology here for humor value) "blur function."